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Saturday, 8 August 2015

Captcha image verification

A good way to avoid automatic form submissions when creating a web form
is to add some kind of verification. One of the best ways is to use an
image verification, called also captcha. What it does is to dynamically
create an image with a random string displayed on it. Then visitor is
asked to type that string in a text field and once the form is submitted
it checks if the string on the image matches the one inputted by the
user. Because there is no easy way to read a text from an image (image
recognition) this is a good way to protect your web forms from spammers.

For doing this CAPTCHA I would suggest using a session variable where
you store the string generated and displayed on that dynamically
generated image.

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<?php

session_start();

$text= rand(10000,99999);

$_SESSION["vercode"] = $text;

$height= 25;

$width= 65;

$image_p= imagecreate($width, $height);

$black= imagecolorallocate($image_p, 0, 0, 0);

$white= imagecolorallocate($image_p, 255, 255, 255);

$font_size= 14;

imagestring($image_p, $font_size, 5, 5, $text, $white);

imagejpeg($image_p, null, 80);

?>

Save this code in a file called captcha.php. What this script does is to
generate a random number from 10000 to 99999 and then assign it to
$_SESSION['vercode']. Then it generates a 25x65 pixels image with black
background and white text using size 14. So if you upload that
captcha.php file on your web site and open
http://www.site.com/captcha.php you will see an image displaying random
integer. You will receive a new random integer every time you refresh
that page.

Next we need to create our web form.

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<form action="submit.php"method="post">

Comment: <textarea name="coment"></textarea>

Enter Code <img src="captcha.php"><input type="text"name="vercode"/>

<input type="submit"name="Submit"value="Submit"/>

</form>

Above code will create a form with a single textarea box, randomly
generated image using the captcha.php script and a text field where you
will have to enter the verification code.

All we have to do now is to make the submit.php script which will check
if the verification code you enter matches the one that has been
randomly generated.